Harry's first clue was that Severus never blushed.
"Are you okay?" Harry asked warily.
"I am... feeling indisposed," Severus admitted.
"You're flushed and," Harry placed a hand on Severus' forehead, "running a fever."
"There are potions..." Severus started.
"I know exactly where they are," Harry interrupted. "After you get into bed."
Severus grumped through the process, but gave a sigh of relief when he lay between cool sheets.
"Here," Harry handed Severus an Anti-fever potion and a mild Sleeping draught.
"The rest will do you good," Harry murmured.
"Shh..." Harry whispered. He climbed in and wrapped himself around Severus.